The present inventor relates to a semiconductor light-emitting element using a compound semiconductor material, particularly, to a short wavelength semiconductor light-emitting element using a GaN-based compound semiconductor material.
Gallium nitride (GaN), which is a III-V group compound semiconductor containing nitrogen, exhibits a large band gap of 3.4 eV, is of a direct transition type and, thus, is expected to provide a good material of a short wavelength light-emitting element. However, it is difficult to find a high quality substrate having the lattice matching that of GaN. In many cases, a GaN crystal is grown on a sapphire substrate for the sake of convenience. In this case, since the lattice mismatch between sapphire and GaN is as large as about 15%, the GaN crystal tends to grow in an island shape. Also, if the GaN layer is thickened in an attempt to achieve growth of a high quality GaN layer, serious difficulties are brought about by a difference in thermal expansion coefficient between the sapphire substrate and the grown GaN layer. Specifically, when cooled, dislocation is increased, or the grown GaN layer is cracked. Under the circumstances, it was difficult to grow a high quality GaN film on a sapphire substrate.
In order to alleviate the detrimental effects produced by the lattice mismatch between the sapphire substrate and the grown GaN layer, a very thin buffer layer is formed in general on the sapphire substrate, followed by growing a GaN layer on the buffer layer. The buffer layer is formed by a crystal growth method under low temperatures using an amorphous or polycrystalline AlN or GaN. In this method, the amorphous or polycrystalline layer is considered to alleviate the thermal strain so as to permit the microcrystals contained inside the buffer layer to form collectively a seed crystal having the directions of the original microcrystals aligned in the heating step at a high temperature of 1000.degree. C., leading to an improved crystal quality.
However, a breakthrough dislocation of a high density caused by the lattice mismatch remains to be eliminated in the GaN layer formed on a buffer layer. In the case of growing a GaN-based compound semiconductor layer such as an AlGaN layer or an AlGaInN layer in a thickness of 0.5 .mu.m or more, said GaN-based compound semiconductor layer having an Al content of at least 15%, which is required for forming a cladding layer of a semiconductor laser, it was unavoidable for the GaN-based compound semiconductor layer to bear cracks caused by residual strain. Where the thickness of the AlGaN layer or AlGaInN layer is 0.2 .mu.m or less, clear cracking is not recognized. However, fine cracks are recognized when observed with an interatomic force surface microscope. These fine cracks act as. electrical resistance where the substrate is made of an insulating material and where current is conducted in a direction parallel to the compound semiconductor layer, leading to deterioration of life characteristics of the semiconductor element.
Particularly, in the case of using as a substrate SiC having a lattice constant close to that of GaN, the movement of the dislocation is slow because the strain is small at the hetero interface, leading to a large residual strain. Also, since the SiC substrate tends to peel more easily than the sapphire substrate, the SiC substrate itself tends to bear cracks.
As described above, it is known to the art to manufacture a semiconductor light-emitting element emitting light of a short wavelength by growing a GaN-based compound semiconductor layer on a single crystalline substrate such as a sapphire substrate or a SiC substrate. In the conventional technique, however, the lattice mismatch between the substrate and the semiconductor crystal layer causes the grown semiconductor crystal layer to bear residual strain, resulting in failure to grow a high quality crystal. Particularly, it is difficult to form an AlGaN or AlGaInN layer having a high Al content and a small residual strain, leading to deterioration in the characteristics of the semiconductor light-emitting element emitting light of a short wavelength.